Certain communication systems use orthogonal frequency division multiplexing (OFDM), sometimes referred to as discrete multi-tone (DMT) transmission, such as those defined by Data Over Cable Service Interface Specification (DOCSIS) 3.1, although the inventive embodiments are not limited to any specific defined protocols.
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry information on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation (QAM) or phase-shift keying (PSK) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
Physical layer (PHY) signaling information is often embedded within the OFDM signal so a receiver can understand the properties of the received signal and be able to properly demodulate and decode the received signal.
For example, in DOCSIS 3.1, PHY signaling information is carried by a narrow in-band channel known as the Physical Layer Link Channel (PLC). A receiver receiving an OFDM signal needs to decode the PLC channel initially and using that information, the receiver may properly decode the main channel carrying data and other signaling components. Unless predetermined, until the PLC is decoded, the receiver has no knowledge of the frequency location of the PLC or other OFDM channel conditions used by the transmitter.
In certain systems like DOCSIS 3.1, a receiver has to apply its OFDM FFT to decode the PLC without necessarily having any knowledge of its positioning by the transmitter IFFT. This is referred to herein as “blind decoding.” In blind decoding, the transmitter IFFT and the receiver FFT will usually not be aligned in frequency during PLC decoding which may give rise to phase discontinuity between successive symbols in a received OFDM signal, and hence an additional algorithm is needed for PLC decoding. The PLC could be pre-determinately located in specific locations in frequency for a receiver to not experience this discontinuity, but that may require standardization or that different manufacturers or service providers to collaborate on a pre-existing configuration to do so first, which is not always possible or desirable. Thus there is a need to be able to “blindly” decode the PLC reliably, regardless of its frequency location by a transmitting device.